The present invention relates to a telecommunications system comprising a supply circuit for a telecommunications line comprising a first and a second wire, which supply circuit comprises a first electronic impedance circuit including a first transistor and a second electronic impedance circuit including a second transistor, the electronic impedance circuits being coupled between a first voltage reference terminal and the first wire and between a second voltage reference terminal and the second wire, respectively. Such a system can be a private automatic branch exchange coupled to a number of subscriber sets via twisted pairs, whereby the sets can be analog or digital sets and whereby the branch exchange can be coupled to the public switched telephone network, or any other suitable system wherein subscriber sets have to be fed via the telecommunications line.
The present invention further relates to an automatic branch exchange, a line card, a supply circuit for a telecommunications line, and a telecommunications subscriber device.
A telecommunications system of the above kind is known from the Belgium Patent No. 1007208A3, filed by the same applicant. This Belgium Patent discloses a telecommunications system comprising a supply circuit for feeding power to a twisted pair telecommunications line to another end of which a telecommunications subscriber device such as a telephone device or a facsimile apparatus or a data terminal, or the like, can be coupled. The telecommunications device is fed via a DC-voltage injected into the twisted pair by means of the supply circuit. The known supply circuit applies line voltage control so that the DC-line current is dependent on the supply needs of the subscriber device. Herewith, power is not unnecessarily dissipated by the supply circuit as would be the case with current control and varying telecommunications line lengths. In the latter case, the supply circuit would have to be designed for a maximum line length and power would be wasted when applying a shorter line length. In order to inject DC-power to the telecommunications line and at the same time to avoid that signal power is fed from the wired to the DC-supply, the known circuit applies electronic impedances symmetrically arranged between the wires and respective DC-power supply lines. The electronic impedances simulate large coils. This is because in modern branch exchanges, or the like, large coils should be avoided. Such coils are expensive and are bulky so that mounting is cumbersome. The electronic impedances comprise a FET (Field Effect Transistor) of which a control electrode is coupled to a DC-supply terminal and a wire via a junction of a series arrangement of a resistor and a capacitor, and further a resistor in the source lead of the FET. Due to the series arrangement of the resistor and the capacitor the gate voltage of the FET can only vary slowly so that a virtually constant voltage occurs across the source resistor of the FET. An AC-voltage at the drain of the FET is attenuated by a factor inversely proportional to the gain of the FET. As a result, the electronic impedance behaves like a lump element coil, the equivalent impedance being equal to the value of the source resistor of the FET multiplied by the gain-factor of the FET, and the DC-resistance virtually being equal to the value of the source resistor. The resistor in the series arrangement preferably has a relatively high value, this resistor being connected parallel to the emulated coil. In order to create an optimal dynamic operating range for the FET and in order to avoid signal distortion and demodulation of strong amplitude modulated disturbance signals such as signals from strong AM-broadcasting transmitters, a diode-resistor series arrangement is provided between the control electrode of the FET and a DC-supply terminal or a wire, as the case may be. Although this electronic impedance circuit operates satisfactorily for relatively low line currents, problems arise when relatively high line currents are needed. Such relatively high DC-line currents, in the order of 200 mA, for instance, are needed when applying multifunctional telecommunications subscriber devices that can include a switched mode power supply for feeding internal circuitry and for feeding external devices. At the other end of the line the line voltage should be relatively low. When applying switched mode power supplies that require a constant power, the line current increases if the line voltage decreases. Furthermore, at the side of the branch exchange, the line voltage typically is -48 Volts but in case of a mains interruption, when accumulators are used, such line voltage could drop to -42 Volts. Under the above circumstances it is important that the voltage losses across the electronic circuits that replace lump element coils are as small as possible. The known circuit comprises a resistor that is coupled between a source electrode and a wire or a DC-supply terminal, as the case may be.
This resistor should be large enough so as to achieve that the impedance of the electronic circuit is as large as possible and small enough so as not to cause a too large voltage drop. When relatively large line currents are requested such a compromise is difficult to achieve without unsatisfactorily system operation.
The French Patent Application No. 2 254 168 discloses a supply circuit for a telecommunications line comprising a Darlington transistor pair instead of a FET. The Darling pair, connected as an electronic impedance, has different electronic properties than a single FET, and has a higher amplification factor. Such a Darlington configuration could be applied for switching on/off relatively high currents but the Darlington pair would not perform well for linear applications or for putting digital data on the line having a relatively high frequency, such as in the order of 40-200 kHz. This is because a Darlington is a current to current amplifier in which a main current is built up quickly after applying a control current to the Darlington, but this main current decreases very slowly if it is being switched off. I.e., the Darlington quickly opens (high gain) but slowly closes. Due to the high loop gain, the Darlington pair will easily become instable, i.e., the circuit will tend to oscillate or exhibit a phenomenon called `motorboating`.